There are conventionally known diesel engine emission-control systems in which catalytic materials including oxidizing catalysts and nitrogen oxides (NOx) reducing catalysts are coated or deposited on a cordierite honeycomb carrier to convert harmful pollutants including hydrocarbons (HC), carbon monoxides (CO), soluble organic fraction (SOF) in particulate matters (PM), nitrogen oxides (NOx), and so on in the exhaust gases into harmless materials. With the conventional catalytic emissions-control systems constructed as stated earlier, the exhaust gases are allowed passing over a large surface areas inside the honeycomb carrier in a manner reversed in flow directions by alternate closing of any one of forward and aft ends of gas passages in the honeycomb carrier to get the particulate matters trapped to purify the exhaust gases. There are two types of the honeycomb carrier, one of which is made to close alternately either of the forward and aft ends of the gas passages inside the honeycomb to trap only the particulate matters contained in the exhaust gases while the other is constructed the gas passages inside the honeycomb remain open to get the exhaust gases flowing in steady direction to exclude harmful materials from the exhaust gases.
Moreover, the diesel engines are recognized once again in recent years for the source of power because their less carbon dioxides (CO2) emission and find increased applications in a diversity of passenger vehicles. Most diesel engines for automobiles recently have diesel-particulate filters (DPF) in which particulate matters trapped inside the ceramic honeycomb are catalyzed at relatively low temperature to get burned off to accomplish the purification of the exhaust gases. As most components of particulate matters contained in the automobile exhaust emissions are unburned carbons or hydrocarbons originated in heavy oil, it will be easily considered complete combustion of fuel through with the DPF helps expel all particulate matters. The catalysts commonly carried on the filters, as heated up to around 300° C., starts to get soot burning or nitrogen monoxides (NO) reducing into nitrogen dioxides (NO2), which is in turn utilized.
With the ceramic honeycombs, especially constructed to remain open at their forward and aft ends as stated earlier, much effort is made to reduce cellular walls of honeycomb in thickness to increase cell density (number of cell per square inch) in the honeycomb. The carrier of honeycomb in practical use in this point has cells of from 200 to 400. In contrast to the ceramic honeycomb as stated earlier, other carrier of metallic honeycomb is in practical use for the catalytic exhaust emission purification. The metallic honeycomb has the construction that corrugated thin sheets and flat sheets stacked alternately on top of each other are wound up together in spiral geometry. Conventional practice in metallic honeycomb constructed as stated earlier is to make the sheets as thin as possible in thickness with keeping mechanical strength to increase number of cells in the honeycomb thereby making sure of large surface areas coming into contact with the exhaust gases.
Meanwhile, it is inevitable to purify the exhaust gases emitted from not only new cars but used cars to keep urban atmosphere clean. Addition of the DPF to the used cars is positively encouraged according to the aspect stated just above. Nevertheless, the diesel-particulate filter added to the currently existing automobiles, because of uncontrollable in unification or association with the engines, often fails to work in good performance.
What is made of composite materials of metallic sheets with metallic nonwoven layers starts recently finding applications in filters for exhaust-gas purification. With the filters constructed as stated earlier, the metallic sheets serve introducing the exhaust gases to come into collision against the metallic nonwoven layers to trap the gasborne particulate matters. These filters are sophisticated in construction and further have a major drawback the decreasing rate of PM is from 40 to 50% at the most and can't be raised any further. Although increase of surface areas to come into contact with the exhaust gases is also beneficial for the metallic honeycombs, there is further proposed the metallic honeycomb that is designed to cause any turbulent flow in the exhaust gases flowing through gas passages defined inside cells in the honeycomb to encourage more contact of the exhaust gases with the catalysts. One example of the metallic honeycombs is disclosed in “Next generation catalysts are turbulent: Development of Support and Coating: opened to public on 2004-01-1488, refer to Internet Google. With the metallic honeycomb recited earlier, the metallic plates thin in thickness are perforated. The perforations made in the metallic plates, though costly to the increase of large surface areas, contribute to generation of turbulent flow inside the gas passages to encourage more contact of the exhaust gases with the catalysts.
In Japanese Laid-Open Application H08-196 918, there is disclosed a catalyst carrier for exhaust-gas purifier in which a thin web of catalyst carrier is received while wound into multiple carrier winds inside a cylindrical casing that remains open at both axial opposite ends thereof. The catalyst carrier fitted inside the casing has turbulent generators, which are made with perforations or deformed surface areas to disturb the flow of exhaust gases. The catalyst carrier is held inside the casing with retainers to keep the catalyst carrier against falling away out of the casing. Thus, the turbulent flow of the exhaust gases inside the cylindrical casing is encouraged to react with catalysts over much time to improve purification efficiency.
Another catalyst carrier for exhaust-gas purification is disclosed in Japanese Laid-Open Application H06-2 536, which has a cylindrical casing, upstream first honeycomb and a downstream second, the first honeycomb being carried on a support member extending axially upstream side beyond the second honeycomb in a way isolated away from the casing.
A metallic carrier for exhaust-gas purification to carry thereon catalysts to purify exhaust emissions including automobile emissions, and so on is disclosed in Japanese Laid-Open Application H01-123 638, in which there is provided a honeycomb construction made of metallic even sheets and corrugated sheets laid alternately on top of each other in the shape of either spiral or laminate, the honeycomb being separated into upstream and downstream sections with respect to the exhaust-gas flow. The honeycomb sections are arranged in series in a way their even sheets overlap one another along their confronting edges so as to get their corrugations somewhat staggered each other. A canister fits over the honeycomb to cover around the outside surface of the honeycomb in a way extending in the direction of internal pore surface areas in the honeycomb. Moreover, radial plates are secured to the axially opposite ends of the canister to keep the honeycomb inside the canister.
Another exhaust-gas purifier is disclosed in Japanese Laid-Open Application H10-159 552, which is envisaged decrease or exclusion of soot particles entrained in the exhaust gases emitted out of the diesel engines. The exhaust-gas purifier includes a first catalytic material for oxidizing NO to NO2, and second catalytic material for oxidization of hydrocarbons, carbon monoxides and volatile organic components. The soot particles trapped on or in the monolith coated with the second catalytic material are burned off with gases containing NO2 coming from the first catalytic material. The monolith for the first catalytic material is designed to keep capture of soot particles to a minimum.
A filter to purify the exhaust emissions coming from the diesel engines is disclosed in International Publication WO2004/015 251, which is composed of gold foil and a web of filtering medium made of a substance allowing fluids to pass in part through there. The filtering medium includes active catalyst coatings to convert gaseous components contained in the exhaust gases and filtering areas to separate and remove particles from the exhaust gases.
A further another exhaust-gas purifier with heating elements is disclosed in International Publication WO01/020,142, which has an outside enclosure to accommodate therein a honeycomb of catalyst carrier allowing exhaust gases to flow through there, and an electric heating element having electric connecting terminals at opposite ends and containing therein clearances for electric insulation to define serpentine flow passages. The honeycomb for the heating element is held in the catalyst carrier by means of electrically insulated carrying elements. The honeycomb for the heating element is placed downstream in respect of the flowing direction of the exhaust gases.
In diesel engines, meanwhile, as there is universally trade-off relation between NOx and PM, it is very tough to decrease them alike. Moreover, the diesel engines, because of rich in oxygen concentration in the exhaust gases, are difficult to reduce the NOx by means of the three-way catalytic converters, which are proved effective in most automotive gasoline engines. Further, soot mainly occupied by PM, because of high in oxidation temperature, needs any means designed specially for decrease thereof. With the existing DPF having filters coated with catalysts, the catalysts of platinum and so on to spare are universally carried on the filters in advance to cope with degradation of the catalysts so as to ensure safely their performance. Nevertheless, even the DPF engineered with the degradation of catalysts in mind as stated earlier might be insufficient for cars running very long distances and help little in keep the intrinsic performance to exclude the PM out of the exhaust gases.
The PM-based soot burns off after trapped in the filters. However, when the cars having mounted with the DPF are forced to drive continuously at slow velocity over time in for example traffic jams, chances the temperature of exhaust gases emitted from the engines reaches burning temperature might be too few in frequency. As a result, the filters are clogged up with much soot to choke the exhaust manifold. Thus, there is possibility that the cars couldn't run any further. In contrast, when the exhaust temperature rises above the combustion temperature of much soot deposits on the filters depending on any changes in driving condition, the PM would burst abruptly to cause filter damages or meltdown and, in some instances, blazes. To cope with this, the advanced DPF is added with a pressure sensor or the like to monitor the amount of soot deposits over the filters, and any means to raise the exhaust temperature to promote combustion of the soot after the amount of soot deposits has been beyond a preselected level.
Among systems to elevate the exhaust temperature in the engines with exhaust-gas purifier are known the system in which a common-rail injection system atomizes the fuel in the combustion chamber during either a latter portion of the expansion phase or the exhaust phase to raise the exhaust temperature, another system in which the an injected fuel is subjected to exothermic oxidation in oxidizing catalysts installed backward, and a further another system in which a different fuel delivery line is provided upstream the PM excluder to feed the fuel into the oxidizing catalysts installed backward to encourage exothermic oxidation to raise the exhaust temperature. With the PM purifiers advanced as stated earlier, there is far less risk than ever of the possibility the filters might be clogged with much soot. Nevertheless, the system to elevate the exhaust temperature with no avail fuel consumption, as activated every time when the exhaust temperature falls short of the burning temperature of the PM at low velocities, has the shortcoming of getting the fuel efficiency worse. With the PM catalysts low in activated temperature, especially, the more the system to elevate the exhaust gases works in frequency, the worse the fuel efficiency is.